An R—Fe—B based rare-earth magnet (where R is one of the rare-earth elements including Y, Fe is iron, and B is boron) is a typical high-performance permanent magnet, has a structure including, as a main phase, an R2Fe14B phase, which is a tertiary tetragonal compound, and exhibits excellent magnet performance.
Such R—Fe—B based rare-earth magnets are roughly classifiable into sintered magnets and bonded magnets. A sintered magnet is produced by compacting a fine powder of an R—Fe—B based magnet alloy (with a mean particle size of several μm) with a press machine and then sintering the resultant compact. On the other hand, a bonded magnet is produced by compacting a mixture (i.e., a compound) of a powder of an R—Fe—B based magnet alloy (with particle sizes of about 100 μm) and a binder resin within a press machine.
The sintered magnet is made of a powder with relatively small particle sizes, and therefore, the respective powder particles thereof exhibit magnetic anisotropy. For that reason, an aligning magnetic field is applied to the powder being compacted by the press machine, thereby obtaining a compact in which the powder particles are aligned with the direction of the magnetic field.
In the bonded magnet on the other hand, the powder particles used have particle sizes exceeding the single domain critical size, and normally exhibit no magnetic anisotropy and cannot be aligned under a magnetic field applied. Accordingly, to produce an anisotropic bonded magnet in which the powder particles are aligned with particular directions, a technique of making a magnetic powder, of which the respective powder particles exhibit the magnetic anisotropy, needs to be established.
To make a rare-earth alloy powder for an anisotropic bonded magnet, an HDDR (hydrogenation-disproportionation-desorption-recombination) process is currently carried out. The “HDDR” process means a process in which the hydrogenation, disproportionation, desorption and recombination are carried out in this order. In this HDDR process, an ingot or a powder of an R—Fe—B based alloy is maintained at a temperature of 500° C. to 1,000° C. within an H2 gas atmosphere or a mixture of an H2 gas and an inert gas so as to occlude hydrogen. Thereafter, the hydrogenated ingot or powder is subjected to a desorption process at a temperature of 500° C. to 1,000° C. until a vacuum atmosphere with an H2 partial pressure of 13 Pa or less or an inert atmosphere with an H2 partial pressure of 13 Pa or less is created. Then, the desorbed ingot or powder is cooled, thereby obtaining an alloy magnet powder.
An R—Fe—B based alloy powder, produced by such an HDDR process, exhibits huge coercivity and has magnetic anisotropy. The alloy powder has such properties because the metal structure thereof substantially becomes an aggregation of crystals with very small sizes of 0.1 μm to 1 μm. More specifically, the high coercivity is achieved because the grain sizes of the very small crystals, obtained by the HDDR process, are close to the single domain critical size of a tetragonal R2Fe14B based compound. The aggregation of those very small crystals of the tetragonal R2Fe14B based compound will be referred to herein as a “recrystallized texture”. Methods of making an R-Fe-B based alloy powder having the recrystallized texture by the HDDR process are disclosed in Japanese Patent Gazettes for Opposition Nos. 6-82575 and 7-68561, for example.
However, if an anisotropic bonded magnet is produced with a magnetic powder prepared by the HDDR process (which will be referred to herein as an “HDDR powder”), then the following problems will arise.
A compact, obtained by pressing a mixture (i.e., a compound) of the HDDR powder and a binder resin under an aligning magnetic field, has been strongly magnetized by the aligning magnetic field. If the compact remains magnetized, however, a magnet powder may be attracted toward the surface of the compact or the compacts may attract and contact with each other to be chipped, for example. In that case, it will be very troublesome to handle such compacts in subsequent manufacturing process steps. For that reason, before unloaded from the press machine, the compact needs to be demagnetized sufficiently. Accordingly, before the magnetized compact is unloaded from the press machine, a “degaussing process” of applying a degaussing field such as a demagnetizing field, of which the direction is opposite to that of the aligning magnetic field, or an alternating attenuating field to the compact needs to be carried out. However, such a degaussing process normally takes as long a time as several tens of seconds. Accordingly, in that case, the cycle time of the pressing process will be twice or more as long as a situation where no degaussing process is carried out (i.e., the cycle time of an isotropic bonded magnet). When the cycle time is extended in this manner, the mass productivity will decrease and the manufacturing cost of the magnet will increase unintentionally.
As for a sintered magnet on the other hand, even if the compact thereof is not degaussed sufficiently, the compact remains magnetized just slightly, because its material magnet powder has low coercivity from the beginning. Also, in the sintering process step, the magnet powder is exposed to an elevated temperature that is higher than the Curie temperature thereof. Thus, the magnet powder will be completely degaussed before subjected to a magnetizing process step. In contrast, as for an anisotropic bonded magnet, if the compact thereof remains magnetized when unloaded from the press machine, then the magnetization will remain there until the magnetizing process step. And if the bonded magnet remains magnetized in the magnetizing process step, the magnet is very hard to magnetize due to the hysteresis characteristic of the magnet.
In order to overcome the problems described above, a main object of the present invention is to provide a method and a press machine for producing an easily magnetizable permanent magnet (e.g., an anisotropic bonded magnet among other things) at a reduced cost by avoiding the problems caused by the unwanted remanent magnetization of the compact.